The invention relates to a hole forming and selftapping screw of up to approx. 6 mm thread diameter for tapping into sheet metal of a thickness of up to approx. 1 mm comprising a head including a formation for fitting a tool, a thread shank and a cylinder member following thereon having a diameter less than the thread-pitch diameter, merging into a tapered hole forming member.
Such a screw is known from DE-PS 25 37 446. In the case of this screw denoted as a selftapping sheet metal screw, the hole forming member serves to drill the hole which will later accommodate the screw, by a chipforming cutting action for which purpose the hole forming member terminates in a distinct point, forming the tip of a pyramid the edges of which serve for the cutting of the relevant sheet metal part. According to FIG. 18 of this patent specification a cylinder member, the diameter of which equals or is less than the thread-pitch diameter of the threads, is provided between the wholly cylindrical thread shank and the hole forming member of a pyramidal configuration. The purpose of this cylinder member is to adapt the drilled hole to the cylinder form and to adjust its axial direction vertically to the hole, thus enabling the thread shank to be inserted into the sheet metal without malalignment.
A further hole forming and selftapping screw is known from DE-PS 27 32 695. This screw is also based on the principle of first penetrating into the material of the sheet metal by means of a sharp point of its conical hole forming member and to drill a pilot hole into the sheet. The hole forming member of this screw is furthermore provided with a thread of large pitch from which during further penetration of the screw into the sheet metal a reaming of the pilot hole, initially drilled by the sharp point, is to be attained by expansion in a flaring manner. The thread of the thread shank in this screw merges directly with the hole forming member.
Various disadvantages result from the fine chipcutting of the sheet metal resulting from the tapping action of the known screws. Fine metal chips are deposited inside casings and the like, where they may, for example in the case of electrical appliances, form undesirable and dangerous contact bridges. Furthermore fine chips can only be removed with difficulty and result in undesirable corrosion, particularly in the presence of moisture or humid air.
It is the object of the invention to so design the screw described in the introduction that the chipforming problem is avoided and a particularly firm fit of the tapped screw in the sheet metal is provided.
According to the invention this is attained in that the thread shank merges with the cylinder member via a taper extending over approximately four threads, a distance corresponding approximately to the four-fold sheet metal thickness, that the hole forming member terminates in a convexly curved friction surface corresponding to a radius of curvature of approximately 0.5 mm and that the entire screw is formed in one piece of tempered steel having a tensile strength of up to approximately 1400 N/mm.sup.2, respectively mechanically hardened steel.
The screw according to the invention creates the hole in the sheet metal by friction of the convexly curved friction surface against the sheet metal which is thereby heated such that the material passes into its plastic range, wherein it becomes radially yielding such that a desirable hole expansion results with material displacement to a nozzle shape. It was found in this context that with a radius of approx. 0.5 mm of the convex curvature of the friction surface a favourable centering ability, sufficiently rapid heat generation combined with high penetration velocity and good heat flow through the screw result. With further tapping of the screw its cylinder member enters into the nozzle, thereby terminating the forming of the hole with the nozzle. Now the tapered portion of the thread shank engages into the hole whereby the female thread is shaped in the hole duly assisted by the taper. The cylinder member moreover permits a gradation so to speak of the rotational speed of the screw to be tapped, because for the initial forming of the hole by friction a substantially higher rotational speed is required than for forming the thread. It is thus avoided that approximately towards the end of the hole forming process the threads of the tapered portion of the thread shank have to engage already.
The heating up of the sheet metal due to friction generated by the convexly curved friction surface depends on the force by which pressure is applied to press the friction surface against the sheet metal. The pressure required for a sufficient heating can be reduced if the sheet metal is provided with a conical depression at the locality where the hole is formed. In that case and if the taper of the cone wall is such that the area of contact of the convexly curved friction surface is limited to the region of the cone wall, the convexly curved friction surface is so designed that the latter corresponds to a radius of approximately 0.5 to 1 mm.
The processing of sheet metal usually requires an initial cutting of the latter to certain dimensions often combined with center punching for bores to be applied thereafter. Therefore, within the scope of such preparatory measures, the provision of a conical depression does not involve particular efforts, especially since the conical depression is not a perforation, involving a punching process with waste formation. Contrary to the above described case of placing the screw with its convexly curved friction surface onto the level sheet metal, the area of contact required for heating up can, due to the conical depression, be increased considerably, namely annularly in the cone wall region, whereby, as compared with applying the screw onto a level metal surface, a substantial reduction of the required pressing force of the screw against the sheet metal becomes possible. This reduction of the pressing force is also favourably influenced by increasing the radius of the convexly curved friction surface because the radius of the annular contact zone and thus its length is expanded correspondingly.
Due to this design of the screw according to the invention a particularly large release moment results because the heated nozzle at the end of the tapping process shrinks around the relatively colder thread shank. Thus a particularly desirable locking effect is attained for the tapped screw.
Tools for forming a hole in sheet metal which heat the metal by friction and due to conical shape of their front end generate a hole with nozzlelike extensions, are e.g. known from DE-AS 25 52665 and DE-AS 28 02 229. These tools comprise a spike, rapidly rotatable around its axis, the spike being placed onto the metal by way of a centering point or spike tip respectively, and the sheet metal being so plastified by the heat of friction that the conical portion of the tool following on to the point can effect an expansion of the hole up to the maximum diameter of the cone which in turn is followed by a cylindrical spike portion. It is also known from FR-PS 1 189 384 to round off the foremost region of such a cone.
The technique of these tools has up to now not been applied to hole forming and selftapping screws. The reason was apparently based on fears that the considerable heating of the tool portion contacting the metal endangers the tool and may cause its destruction. For this reason the tools which became known in practice, so-called tile drills, consist of heat-resistant materials, in particular hard metal. Such materials cannot be considered for the present type screws for cost as well as technical reasons, since they will above all be exposed to considerable tensile stresses for which hard metal in particular is not suitable. It now appears that in sheet metals, in particular steel sheet up to a thickness of 1 mm holes can be formed using a hole forming member comprising the aforesaid convexly curved friction surface, by exploiting the considerable frictional heat generated thereby, if for the screw as a whole tempered steel having a tensile strength of up to approximately 1400 N/mm.sup.2, or mechanically hardended steel respectively is used. Both materials are highly suitable for use as screws due to their tensile strength. When forming the hole in the sheet metal a high frictional heat results due to the convexly curved friction surface which is not readily dissipated by the relatively thin sheet metal, causing a heat build-up in the sheet metal favouring its plastification. Because of the relatively small thickness of the sheet metal this involves practically no mechanical impairment yet of the convexly curved friction surface so that the hole is formed so rapidly that the friction surface is preserved in practice. Even if the friction surface were to be thus impaired, this would be practically of no importance for the penetration of the screw because the forming of the hole by the screw is only necessary once and the subsequent forming of the thread is effected substantially without problems.
Advantageously the hole forming member is shaped in cone-like manner. The cone, due to its geometry, takes care of a continuous material feed both in radial and axial direction.
In this context it may be desirable to reduce the friction of the cone surface against the sheet metal which is attained in that the cone comprises a plurality of symmetrically arranged flattened regions which in cross section merge with one another via conically rounded regions. The deforming of the sheet metal is thus facilitated.
The material deformation is further facilitated in that the cone and the flattened regions merge with the cylinder member via an axially rounded region. Due to the axially rounded region a uniform expansion of the material to be deformed results until the cylinder member is reached, and thus a favourable material flow. The hole forming member of the screw may thus advantageously be so designed that the cone angle averages approximately 30.degree. to 40.degree..
The formation of the nozzle formed by plastic deformation permits the forming in the nozzle of female threads of a pitch substantially smaller than the usual pitch in known so-called sheet metal screws.
The screw may thus advantageously be so designed that the pitch P of the thread in relation to its outer diameter d corresponds to the formula P=0.15 to 0.20 d.
If it is desired not to crush this collar when tightening the screw, the head of the screw is advantageously so designed that the latter at its underside facing the thread shank comprises an annular groove the inner wall of which has a radius corresponding substantially to the radius of the thread flanks and which accommodates the front end of a nozzle formed during the penetration of the screw into the sheet metal, that is to say the abovementioned collar.
The crushing of the collar with a screw having a smooth underside of its screw head often results in cutting the collar to a certain extent into chips. Such metal chips are undesirable because of dirtying and subsequent corrosion, when using the screw in connection with electrical appliances, such metal chips may also lead to electrical complications, in particular short circuits.
The annular groove may be given a rectangular cross section. For this purpose the annular groove is pressed into the screw head when the screw is manufactured.
It is also possible to let the outer wall of the annular groove proceed with a slope to the outside from its base. In this case the slanted annular groove can press the collar inwardly when the latter contacts the slanted wall. This design serves particularly to press the collar against the thread for which purpose the annular groove is made of such a depth that the front end of the nozzle, when the screw is tightened, is pressed in the direction of the thread shank. This attains on the one hand a particularly good sealing between the nozzle and the thread of the screw and on the other hand also a locking of the screw resulting in a considerably higher release moment.